here's how Kris Knight can still win

We didn't fight the Knight we fought the Shadow of the Knight's Hand

Kris's severed hand is a darkner , he has a vessel hand replacing it

"a nightmare awakens in the shadow of the knight's hand"

The knight might actually still be Kris , it's their missing hand (that got replaced by the Vessel's gray hand), drawing out different forms like shadow puppets. Said hand may contain Kris's original soul.

The knight we fight is a shadow hand puppet projection by the actual person doing it.

That's why it fades into static when hit and why it transforms into other shapes so easily like the flying bird form it abducts Undyne with.

The real body is in the bunker/depths, it's probably Dess or Gaster projecting it or even Kris's severed hand

(any of these fit with "the shadow of the knight's hand" and "speaking in hands").

Bonus schizo material: Shadow Puppets and Kris's are both Indonesian traditional art forms tied to their religions

The knight we fight is a shadow hand puppet projection by the actual person doing it.

That's why it fades into static when hit and why it transforms into other shapes so easily like the flying bird form it abducts Undyne with.

The real body is in the bunker, it's probably Dess or Gaster projecting it or even Kris's severed hand